Intellgent Traffc Condtoners for Assured Forwardng Based Dfferentated Servces Networks B. Nandy, N. Seddgh, P. Peda, J. Ethrdge Nortel Networks, Ottawa, Canada Emal:{bnandy, nseddgh, ppeda, jethrdg}@nortelnetworks.com Verson 7 ABSTACT Issues related to bandwdth assurance n Assured Forwardng based Dfferentated Servces (Dffserv) networks have been dscussed n recent research papers [7] [8][]. Some of the factors that can bas bandwdth assurance are ound Trp Tme (TT), /TCP nteracton and dfferent target rates. The bas due to these factors needs to be mtgated before bandwdth assurance for a payng customer can be artculated n Servce Level Agreements (SLAs). Ths paper proposes ntellgent traffc condtonng approaches at the edge of the network to mtgate the effect of ound Trp Tme, /TCP nteractons, and dfferent target rates. The smulaton results show a sgnfcant mprovement n bandwdth assurance wth ntellgent traffc condtonng. The lmtaton of the proposed solutons s that they requre communcaton between edge devces. In addton, these solutons are not applcable for a one-to-any network topology.. Introducton The Dfferentated Servces (Dffserv) archtecture [] has recently become the preferred method to address QoS ssues n IP networks. Ths packet markng based approach to IP-QoS s attractve due to ts smplcty and scalablty. An end-to-end dfferentated servce s obtaned by the concatenaton of per-doman servces and Servce Level Agreements (SLAs) between adjonng domans along the source-to-destnaton traffc path. Per doman servces are realzed by traffc condtonng at the edge and smple dfferentated forwardng mechansms at the core of the network. Two forwardng mechansms recently standardzed by the IETF are the Expedted Forwardng (EF) [6] and Assured Forwardng (AF) [5] Per Hop Behavors (PHBs). The bass of the AF PHB s dfferentated droppng of packets durng congeston at the router. The dfferentated droppng s acheved va ED-lke [] Actve Queue Management (AQM) technques. The AF PHB FC specfes four classes and three levels of drop precedence per class. AF s an extenson of the IO [3] scheme, whch uses a sngle FIFO queue and two levels of drop precedence. To buld an end to end servce wth AF, subscrbed traffc profles for customers are mantaned at the traffc condtonng nodes at the edge of the network. The aggregated traffc s montored and packets are marked at the traffc condtoner. When the measured traffc exceeds the commtted target rate, the packets are marked wth hgher drop precedence (DP); otherwse, packets are marked wth lower drop precedence (DP0). If the measured traffc exceeds the peak An abrdged verson of ths paper s accepted for publcaton n Hgh Performance Networkng 000 Conference, May 000, Pars, France
target rate, the packets are marked wth hghest drop precedence (DP). At the core of the network, at the tme of congeston, the packets wth DP markng have hgher probablty of beng dropped than packets wth DP0 markng. Smlarly, packets wth DP markng have hgher probablty of beng dropped than packets wth DP0 and DP markng. The dfferent drop probabltes are acheved by mantanng three dfferent sets of ED parameters one for each of the drop precedence markngs Although the IETF Dffserv Workng Group has fnalzed the basc buldng blocks for Dffserv, we argue that there are many open ssues n understandng and evaluatng the knds of endto-end servces that could be created for an end user usng the AF PHB. Varous ssues wth bandwdth assurance n a Dffserv network have been reported n recent research papers [][]. A number of these ssues need to be resolved before quanttatve assurances of some form can be specfed n SLA contracts. Bandwdth assurance can be mproved by ntellgent treatment of aggregated flows at the core or at the edge of the network. Any approach to mtgate the mpact of ound Trp Tme (TT), TCP/ nteracton or target rate requres state trackng. Mantanng per-flow or per-polcy state nformaton at the core of the network wll cause scalablty concern. Another alternatve s to address the bandwdth assurance ssues at the edge of the network va ntellgent traffc condtonng. The key contrbuton of ths paper s the proposal of ntellgent traffc condtoners to mtgate the effects of varous factors n basng the acheved bandwdth. An TT-Aware Marker based on the Tme Sldng Wndow (TSW) [3] s developed to reduce the effects of TT n determnng the acheved bandwdth for TCP flows. Extensve study s performed to consder whether /TCP farness ssues can be solved va ntellgent mappng of TCP and traffc to dfferent drop precedence or AF classes. Fnally, two Target ate-aware Markers are presented wth the objectve of dstrbutng excess bandwdth n proporton to the target rates. The rest of ths paper s organzed as follows. elated work s examned n the next Secton. Secton 3 descrbes the topology of the test network and varous smulaton parameters. Secton presents the soluton to mtgate the mpact of TT. TCP/ nteracton ssues are addressed n Secton 5. An algorthm for excess bandwdth dstrbuton n proporton to target rates s dscussed n Secton 6. Secton 7 provdes an analyss, dscusson and evaluaton of the proposed solutons. Secton 8 contans concludng remarks and ponts to areas of future work.. elated Work Clark and Fang [3] reported the ntal smulaton study on a dfferentated drop scheme. Ther paper ntroduced IO (ED wth In/Out) and a remarkng polcer that utlzed an average tme sldng wndow (TSW) rate estmator and ntellgent marker. The man contrbuton of that work was to show that source target rates could be assured n a smple capacty allocated network that reles on statstcal multplexng. Ibanez and Nchols [], va smulaton studes, showed that TT, target rate and TCP/ nteractons are key factors n the throughput of flows that obtan an Assured Servce usng a IO-lke scheme. Ther man concluson s that such an Assured Servce cannot offer a quantfable servce to TCP traffc. Seddgh, Nandy and Peda [] have confrmed wth detaled expermental study that the above mentoned factors are crtcal for basng dstrbuton of excess bandwdth n an over-provsoned network. In addton, t has been shown [] that the number of mcro-flows n an aggregate and packet szes play key roles n determnng the bandwdth acheved n over-provsoned networks. ecently, varous researchers [7][][] have reported new approaches to mtgate the basng effects of some of the factors outlned n [] and []. Ln, Zheng and Hou[7] have proposed an
enhanced TSW profler and two enhanced IO queue management algorthms. Ther smulaton results show that the combnaton of enhanced algorthms mproves the throughput and farness requrements especally wth dfferent target rates, TTs and co-exstng flows. However, the proposed solutons may not be scaleable due to the usage of state nformaton at the core of the network. Yeom and eddy [] have suggested an algorthm that mproves farness for the case where the ndvdual flows n an aggregate have dfferent TTs. The proposed algorthm mantans per-flow nformaton at the edge of the network. Km [] proposes a token allocaton scheme to dstrbute tokens to ndvdual flows orgnatng from the same subscrber network. The paper clams that usng ths approach, farness n TCP and nteracton and farness between TCP connectons wth dfferent TTs can be acheved. The detals of the algorthm are not clearly reported n the IETF draft[]. 3. Smulaton Detal The studes n ths paper were performed usng the ns- smulator [5]. The smulator was enhanced to nclude networkng elements wth Dffserv edge and core devce functonalty as specfed n []. The network topology used n the experments can be seen n Fgure. The setup conssts of three network edge devces E, E, E3 and one core devce C. Each edge devce s connected to an end host or traffc source. The TCP flows generated are all long lastng. Experments wth the TT-Aware Traffc Condtoner are performed wth the network topology shown n Fgure. The topology of the network for the experments wth TCP/ nteracton s an extenson of Fgure. Sx edges are connected to sx separate traffc sources. The Target ate-aware Traffc Condtoner also utlzes the same topology wth sx edges and sources. The bottleneck lnk s between core devce C and edge devce E3. E E3 3 C E Bottleneck Lnk - Edge Devce - Traffc Source - Core Devce Fgure : Smulaton Testbed The Edge devces n the testbed classfy packets based on source and destnaton IP addresses. The traffc condtonng scheme s a remarkng polcer that utlzes the Tme Sldng Wndow (TSW) tagger [3] scheme proposed to work n conjuncton wth IO. Ths s referred to as the Standard Traffc Condtoner (TC). The core devce mplements the AF PHB usng the three-colour verson of IO [3]. Three sets of ED thresholds are mantaned n the core devce; one for each drop precedence. Three separate average buffer occupancy or queue length calculatons are tracked: one for DP0 packets (q0), one for DP packets (q) and one for DP (q) packets. The probablty of droppng DP0 packets depends only on the buffer occupancy q0. The probablty of droppng DP packets depends on
the total buffer occupancy of q0 plus q. The probablty of droppng DP packets depends on the total buffer occupancy of q0 plus q plus q. Ths scheme gves the appearance of three coupled vrtual queues wthn a physcal queue. Drop Precedence 0 Drop Precedence Drop Precedence Mn th 0 pkts 5 pkts 0 pkts Max th 55 pkts 0 pkts 5 pkts Max p 0.0 0.05 0. w q 0.00 0.00 0.00 Table : ED Parameter Settngs Careful consderatons need to be gven to the settng of ED parameters for the three dfferent Drop Precedences. The experments n ths paper unless otherwse specfed utlze the ED parameter settngs n Table. The mn th and max th thresholds are selected so that no lower drop precedence packets are dropped tll all hgher drop precedence packets are beng dropped.. Mtgatng the Impact of ound Trp Tme Studes have shown that n Best Effort networks [9], the bandwdth acheved by TCP flows s a functon of the ound Trp Tme (TT). Ths dependency s due to TCP s use of a self-clocked sldng wndow based mechansm. ecent studes [] have shown that flows wth dfferent TTs, despte havng dentcal target rates, wll get dfferent shares of the bandwdth. For overprovsoned networks, the flows wll mostly acheve ther target rate rrespectve of ther TTs[][]. Ths s because DP0 traffc s protected and DP traffc wll be dropped before any DP0 packets are dropped. However, there wll be an unfar sharng of the excess bandwdth n favor of those target aggregates wth lower TTs. In the under-provsoned case, nether of the aggregated flows wll acheve ts target. However, the flows wth a hgh TT wll be further away from the target than the flows wth a low TT. An ntal smulaton s performed to show the mpact of TT on bandwdth and to develop the bass for the TT-Aware Traffc Condtoner. Two traffc aggregates are generated. Each aggregate has target rate of Mbps. Each aggregate (between clent and 3; and between clent and ) has sx TCP flows. Ths profle results n a total allocated bandwdth of Mbps, whch s 0% of the bandwdth at the bottleneck lnk. The transmsson delay between edges E and E3 (TT 3 ) s kept at 0 ms whle TT (between clent and ) s vared from to 00 ms. Comparson of TT ato to Bandwdth ato for Standard TC 3.5 ato.5 0.5 0 0 8 36 5 60 68 76 8 9 00 08 6 Acheved Bandwdth - Standard Traffc Condtoner 8 7 Clent -3 vs Clent - Bandwdth (Mbps) 6 5 Clent -3: TT: 0 ms Clent -: TT: X ms BW ato TT ato 3 TT between Clent and 3 = 0 ms 0 8 36 5 60 68 76 8 9 00 08 6 3 0 8 56 6 7 80 88 96 TT for Aggregate between Clent and (X ms) 3 0 8 56 6 7 80 88 96 TT for Aggregate between Clent and (X ms) Fgure : Acheved BW Usng Standard TC Fgure 3: BW ato Vs. TT ato Fgure shows the total bandwdth acheved by each aggregate. Fgure shows that as the TT between clent and s ncreased, the share of bandwdth of the aggregate decreases. The result reflects the steady state TCP behavor as reported by Maths et al. [9]. Equaton () shows that the BW s nversely proportonal to TT. 3
MSS BW, where MSS s the segment sze and p s packet drop probablty () TT * p As the drop rate and MSS are same for both traffc aggregates, from Equaton () the BW ratos BW can be represented as: TT = 3 () BW 3 TT Fgure 3 plots the rato of TTs and bandwdth from the smulaton results. It shows that the rato of the two TTs s dentcal to the nverse rato of the measured TCP aggregate bandwdth between clents -3 and -, thus verfyng Equaton. Equaton forms the bass of the TT- Aware traffc condtoner. TT-Aware Traffc Condtonng Varous approaches are possble to address the mpact of TT on TCP throughput. One approach s to modfy the TCP wndowng mechansm at the end host and make t TT aware. A second method s to use the knowledge of TT to affect droppng at the congested core devces. A thrd alternatve s to ntroduce a mechansm at the edge of the network to handle the mpact of TT on throughput. We have taken the thrd approach. Equaton shows that f the packet drop rate can be adjusted n relaton to TT, the acqured bandwdth for the aggregate can be made less senstve to TT. Ths dea s the bass of the TT- Aware traffc condtonng algorthm. The aggregates wth hgh TTs take longer to ramp up after a packet drop occurs. Thus, the acheved average bandwdths for hgh TT aggregates are lower. Protectng a hgher amount of traffc for long TT aggregates can compensate for the loss n bandwdth. Our approach ncreases the amount of n-profle traffc for hgh TT aggregates n a proportonal manner. If (measuredate <= Targetate) /*.e., IN-profle */ Map Packets to dp0 Else /*.e., OUT-of-profle */ Map Packets to dp0 wth probablty (-p) Map Packets to dp wth probablty p 8 7 Acheved Bandwdth - TT-Aware Traffc Condtoner Where: p = q * r ( Measureda te Targetate) q = Measuredate Bandwdth (Mbps) 6 5 Clent -3: TT: 0 ms Clent -: TT: X ms r = mn TT aggregatett 3 0 8 36 5 60 68 76 8 9 00 08 6 3 0 8 56 6 TT for Aggregate between Clent and (X ms) 7 80 88 96 Fgure : TT-Aware TC Algorthm Fgure 5: Acheved BW usng TT-Aware TC Fgure outlnes the TT-Aware packet markng algorthm. The algorthm s an extenson of the TSW marker[3]. A detaled dervaton of the algorthm s presented n Appendx A. As long as the measured sendng rate remans below the target rate packets are marked wth DP0. Beyond the target rate, packets are marked DP wth probablty p and DP0 wth probablty (-p). The probablty p s calculated usng knowledge of the traffc stream s measured TT relatve to the mnmum TT (mntt) n the DS doman. For traffc streams wth lower TT, packets beyond the target rate wll get marked to DP wth hgher probablty. At the tme of congeston, more packets wth DP markng wll be susceptble to droppng, thus adjustng the acheved bandwdth. Three assumptons for ths scheme are: (a) all the flows n the aggregate have the same
TT.e., source and destnaton ponts are the same; (b) mntt of the network s known to all the edge devces; (c) TT for the aggregate flow s known at the edge of the network. We repeat the same experment for whch the result was shown n Fgure ; except the TT- Aware TC s used nstead of standard TC. Fgure 5 shows the results. Comparng Fgure 5 wth Fgure, we observe that the mpact of TT has been sgnfcantly mtgated. The two aggregates acheve a smlar share of the excess bandwdth. One major assumpton for the TT-Aware TC s that the TCP flows are operatng n congeston avodance state. Equaton () s not representatve of bandwdth acheved f flows are n slow start. Wth a large number of flows n an aggregate and napproprate settng of ED parameters, many flows can tmeout and enter slow-start repeatedly. In such a case, t has been observed that the TT-Aware TC s less effectve n mtgatng the mpact of TT n basng bandwdth dstrbuton. The next sub-secton dscusses the ssues wth large number of flows and studes the applcablty of the proposed TT-Aware markng algorthm. Issues wth Large Number of Actve Flows The total number of actve flows n the core of the network and the buffer allocaton plays an mportant role [0] n determnng the TCP throughput for ndvdual flows as well as flow aggregates. Large number of actve TCP flows wll cause the queue length to cross the ED max th value and drop multple packets causng tmeout. Smulaton s performed usng the standard TC wth ED parameters as n Table. There are 00 flows per aggregate between clents and 3, and and respectvely. It s observed that wth ncreased TT between clent -, the dfference n aggregate bandwdth between Clents -3, and Clents - are less than that shown n Fgure. The bandwdth does not follow the relatonshp shown n Equaton. Ths s due to the fact that at any gven tme more than 0% of the flows were ncurrng tmeout. When, the TT-Aware traffc condtonng was appled, the mpact of TT dd not get resolved. Agan, the proposed soluton dd not work snce a large number of flows are not n steady state (.e., not obeyng Equaton ). Acheved Bandwdth (Large # of Mcroflows) - TT Aware Traffc Condtoner 8 7 Fgure 6: Acheved BW usng TT-Aware TC (00 mcroflows) Bandwdth (Mbps) 6 5 Clent -3: TT: 0 ms Clent -: TT: X ms 3 0 8 36 5 60 68 76 8 9 00 08 6 3 0 8 56 6 7 80 88 96 TT for Aggregate between Clent and (X ms) Two solutons can be consdered for ths problem. The frst soluton s to develop an TT-Aware TC that takes nto account the possblty of flows enterng tmeout. Ths s dffcult snce some of the flows wll be n slow start and some n congeston avodance state. A second soluton s to use larger buffers. Proper engneerng of ED parameters are key to ths problem. The smulaton s repeated wth sgnfcantly larger buffers.e., very hgh ED mn th and max th. It s observed from Fgure 6 that the TT-Aware TC s capable of compensatng the bandwdth dfferences for large number of flows. 5
5. Addressng Farness ssues wth TCP/ Interactons A payng Dffserv customer wll nject both TCP and traffc to the Dffserv network. The nteracton between TCP and may cause the unresponsve traffc to mpact the TCP traffc n an adverse manner. There clearly, s a need to ensure that responsve TCP flows are protected from non-responsve flows, but at the same tme to protect certan flows whch requre the same far treatment as TCP due to multmeda demands. Moreover, we argue t s the Dffserv customer who should decde the mportance of the payload assumng the network s capable of handlng both TCP and traffc n a far manner. We suggest that three farness crtera for TCP and traffc are:. In an over-provsoned network, both and TCP target rates should be acheved.. In an over-provsoned network, and TCP packets should have a reasonable share of the excess bandwdth. Nether TCP nor should be dened access to the excess bandwdth. 3. In an under-provsoned network, TCP and flows should experence degradaton n proporton to ther target bandwdth. There are two possble approaches to solve the farness ssues: (a) Mappng TCP and to dfferent drop precedence of the same AF class, (b) Mappng TCP and to dfferent AF class queues. Experments are performed wth two sources wth target rates of Mbps and sendng rate of 6 Mbps each CB. Hgh sendng rates of flows are chosen so that the mpact of on TCP can be easly evaluated. Four TCP aggregates are generated wth each aggregate consstng of 3 flows. The target rate of each TCP aggregate s vared from 0.5Mbps to 3Mbps. Thus the total target rates of and TCP aggregates are vared from Mbps to Mbps so that bandwdth allocaton at the core of the network changes from over-provsoned state (0% allocated capacty) to under-provsoned state (0% allocated capacty). Mappng TCP and to Dfferent Drop Precedence A drop precedence mappng scheme s one way to ensure farness for both TCP and. Ths study attempts to evaluate varous optons of mappng TCP and to dfferent drop precedence based on the matrx n Table. In all scenaros, TCP traffc wthn the target bandwdth ( IN- Profle ) s assgned to DP0. In scenaros and 6, n-profle traffc s also assgned to DP0. n-profle traffc assgnment to DP (n scenaros, 3 and ) and to DP (n scenaro 5) are also consdered. The experments are performed wth ntellgent TC to perform approprate mappng at the edge of the network. Table : Possbltes for Mappng TCP and to dfferent drop precedences Scenaros 3 5 6 TCP IN Profle TCP OUT-of Profle IN Profle OUT-of Profle DP0 DP0 DP0 DP0 DP0 DP0 DP DP DP DP DP DP DP0 DP DP DP DP DP0 DP DP* DP DP DP * DP * No dstncton s made between IN and OUT packets 6
In Scenaro, both and TCP n-profle packets are mapped to DP0 and out-of-profle packets are mapped to DP. Both TCP and flows acheve ther target bandwdth n an overprovsoned network (Fgure 7). The flows get most of the share of the excess bandwdth. As the network approaches an under-provsoned state, the TCP flows suffer more degradaton than the flows. Ths s due to dentcal mappng of TCP and out-of-profle traffc. Scenaro : Acheved Bandwdth for TCP and Flows Scenaro 6: Acheved Bandwdth for TCP and Flows.00.5 Drop- Pref Mappng Drop-Pref Mappng 3.50 3.00 TcpIn: TcpOut: UdpIn: DP0 DP DP0 UdpOut: DP.0 Tcp In: DP0 Tcp Out: DP Udp In: DP0 Udp Out: DP Ba.50 nd w dt.00 h (M bp.50 s) Target ates TCP : X Mbps TCP : X Mbps TCP 3: X Mb TCP : X Mb : Mbps : Mb Bandwdth (Mbps).5.0 Target ates TCP : X Mbps TCP : X Mbps TCP 3: X Mbps TCP : X Mbps : Mbps : Mbps.00 0.50 Send@ 6 Mbps 3 Flows/TCP Aggregate Interleaved W 0 Mbps Bottleneck 0.5 Send@ 6 Mbps 3 Flows/TCP Aggregate Interleaved W 0 Mbps Bottleneck 0.00 0.5.5.5 3 Target ate for one TCP Aggregate group (X Mbps) 0.0 0.5.5.5 3 Target ate for one TCP Aggregate group (X Mbps) Fgure 7: Scenaro Fgure 8: Scenaro 6 It s observed that Scenaros to 5 cannot assure target bandwdth. Ths s due to the allocaton of n-profle traffc to DP or DP. Thus, n-profle traffc s dependent on DP0 TCP traffc. Total n-profle TCP traffc determnes f the target rate of can be acheved or not. In other words, the buffer occupancy of n-profle traffc s dependent on the TCP nprofle traffc n DP0. Sharng of excess bandwdth s dependent on the assgned drop precedence of TCP and. Smlar argument s true for under-provsoned scenaro. In Scenaro 6 both TCP and n-profle packets are mapped to DP0. However, TCP out-ofprofle packets are mapped to DP, whle out-of-profle packets are mapped to DP. The results are shown n Fgure 8. In the over-provsoned case, both TCP and acheve ther target bandwdth. However, TCP obtans a greater share of the excess bandwdth than. In an under-provsoned network, the TCP flows experence greater degradaton from ther target bandwdth than the flows. The results show that the target bandwdth for TCP and flows can be acheved by protectng the n-profle traffc and mappng t to DP0. For an over-provsoned network, the manner n whch the excess bandwdth s shared (.e., farness crtera ) remans dependent on the drop precedence assgnment of TCP and out-of-profle packets. In an under-provsoned network (.e., farness crtera 3), solaton of TCP and n-profle traffc s necessary. Mappng both and TCP n-profle to the same drop precedence (.e., Scenaros and 6) results n unfarness to TCP, as t experences degradaton from ts target bandwdth n comparson to. Mappng TCP and to Dfferent AF Class Queues Another way to acheve farness s to completely solate the TCP and traffc n two separate AF class queues at the core of the network. At the edge of the network, the ntellgent TC marks the TCP and packets to dfferent AF classes. A weghted schedulng scheme s used at the core to enforce farness among TCP and flow aggregates. If the weghts of the schedulng class queues are dstrbuted n proporton to the TCP and target rates, the farness crtera can be satsfed. The weghts for the queues can be selected usng followng method: 7
W TCP = n = n = TCP + TCP m = W = where TCP : Target ate for TCP aggregate : Target ate for aggregate W : Weght for Schedulng Queue W TCP : Weght for TCP Schedulng Queue The above equatons set the weght assumng that aggregates are sendng packets at rate equvalent to or greater than ther target rates. TCP traffc and traffc are mapped to dfferent AF classes. IN-profle traffc s mapped to DP0 and OUT-of-profle traffc s mapped to DP. A weghted round robn scheduler s used to schedule packets between two queues at the core of the network. The traffc mx s the same as that used for the results of Fgure 7 and Fgure 8. The results are depcted n Fgure 9. It s observed that all the three farness crtera are satsfed. Both TCP and acheve ther target rates. In over-provsoned case, TCP and obtan a reasonably far share of the excess bandwdth. In the under-provsoned case, the aggregated bandwdth for TCP and degrades proportonally. n = m = TCP + m = Two Queue Soluton: Acheved Bandwdth for TCP and Flows Devaton from Expected Share of Bandwdth 3.0.00 Bandwdth (Mbps).5.0.5.0 0.5 Target ates TCP : X Mbps TCP : X Mbps TCP 3: X Mbps TCP : X Mbps : Mbps : Mbps Send@ 6 Mbps 3 Flows/TCP Aggregate Interleaved W 0 Mbps Bottleneck Percent Devaton from Expected Bandwdth 0.90 0.80 0.70 0.60 0.50 0.0 0.30 0.0 0.0 0.00 3 5 6 q TCP: Over-Provson case TCP: Under-Provson case : Over-Provson c ase : Under-Provson c ase Send@ 6 Mbps 3 Flows/TCP Aggregate Interleaved W 0 Mbps Bottleneck Scenaro 0.0 0.5.5.5 3 Target ate for one TCP Aggregate group (X Mbps) Fgure 9: Scenaro: Two Queue Fgure 0: Devaton from Expected Bandwdth Farness Analyss To further compare the results aganst the orgnal farness crtera, quanttatve analyss s performed. The analyss compares the expected versus the actual bandwdth obtaned for each of the seven scenaros. The percentage devaton from expected share of bandwdth s calculated. These values gve two sets of averages, one for the four TCP aggregates and one for the two aggregates. To facltate detaled analyss, we dstngush between under-provsoned and overprovsoned cases. For the over-provsoned calculaton, we use ponts for case of TCP aggregate target rate wth values 0.5,,.5; for under-provsoned, we use,.5 and 3. Equaton (3) calculates the expected far share of the bandwdth for customers. The maxmum sendng rate of s consdered by takng the mnmum between the stream maxmum sendng rate and the far share bandwdth. 8
Expected Agg BW = Mn BW lnk, S n m j + = TCP j = = Target rate (Mbps) for customer j udp j tcp = Target rate (Mbps) for TCP customer BW lnk = Lnk bandwdth (Mbps) S udp = Maxmum sendng rate (Mbps) for customer udp (3) Equaton (5) determnes the expected far share of the bandwdth for TCP aggregates. Unused bandwdth (Equaton ) from the aggregate(s) s dvded between the TCP aggregates, proportonal to ther target rate. aggregates have unused bandwdth when ther sendng rate s below the target rate. Total Unused BW s gven by: m k m U = k Max BW, 0 k = lnk S k = udp () n m j + = TCP j = Expected TCP Customer BW = TCP ( BW U ) n m lnk + udp TCP + = = (5) Devaton from Expected BW = ExpectedBandwdth MeasuredBandwdth ExpectedBandwdth (6) The graph n Fgure 0 llustrates the results of the quanttatve analyss. From the graph we can see that the test wth two class queues had the least devaton from expected BW. Devaton n Scenaro 6 s also comparable to the test wth two class queues. Scenaro has hgh devaton for excess BW (for both TCP and ). performs poorly for under provsoned cases n Scenaro -5. 6. Excess BW Dstrbuton for Aggregates wth Dfferent Target ates In a Dffserv network, dfferent customers wll contract dfferent target rates. ecent research has shown that n an over-provsoned network, wth standard TC, there s an almost even dstrbuton of excess bandwdth rrespectve of the target rate[]. Ths may not be an acceptable soluton, as the hgh payng customer wth a hgher target rate wll expect a hgher share of the excess bandwdth. Further dscusson on the mert of equal versus proportonal dstrbuton of excess bandwdth can be found n secton 7. Ths work assumes proportonal dstrbuton s desrable. Ths secton descrbes and evaluates two ntellgent traffc condtoners developed to address the ssue of proportonal dstrbuton of excess bandwdth. The frst soluton uses DP0 and DP and 9
s referred to as Target Aware TC wth two drop precedence (TATC-DP). The TATC-DP approach s smlar to the TT-Aware TC. The excess out-of-profle traffc s allocated back to nprofle n proporton to the target rates. Ths wll lead to hgher assured bandwdth for aggregates wth hgh target rate. The algorthm n Fgure outlnes the TATC-D markng scheme for the traffc condtoner. The second soluton uses all three drop precedence and s called TATC-3DP. In ths scheme, the excess bandwdth s dvded between DP and DP n proporton to the target rate. The algorthm for the TATC-3D s captured n Fgure. If (measuredate <= Targetate) /*.e., IN-profle */ Map Packets to dp0 Else /*.e., OUT-of-profle */ Map Packets to dp0 wth probablty (-p); Map Packets to dp wth probablty p; Where: p = q * r : q = ( Measureda te T argetate) Measuredate mntrgetate r = aggregatetrgetates If (measuredate <= Targetate) /*.e., IN-profle */ Map Packets to dp0 Else /*.e., OUT-of-profle */ Map Packets to dp0 wth probablty (-p); If (packet s not marked dp0 ) Map Packets to dp wth probablty -q; Map Packets to dp wth probablty q; Where: p and q: p = q = ( Measureda te Targetate) Measuredate mn Targetate AggregateTargetate Fgure : The TATC-D Algorthm Fgure : The TATC-3D Algorthm We perform the same set of experments usng both the TATC-DP and the TATC-3DP. The frst experment s performed wth two sets of aggregates from clents to 3 and to respectvely. Each aggregate conssts of sx TCP flows. One aggregate has a target rate of Mbps and the other aggregate has a target rate that s vared between 0.5 to.5 Mbps; thus creatng a capacty allocaton from 5% to 0% at the bottleneck lnk. Acheved Bandwdth: Target Aware Traffc Condtoner - Drop Precedence (TATC -DP) Acheved Bandwdth: Target Aware Traffc Condtoner - 3 DropPref (TATC-3DP) 0.0 0.0 9.0 9.0 Bandwdth (Mbps) 8.0 7.0 6.0 5.0.0 3.0.0.0 0.0 0.5.5.5 3.5.5 5.5 6.5 7.5 8.5 9.5 0.5.5 TCP : Standard TC TCP : Target Aware TC TCP : Ideal TCP : Standard TC TCP : Target Aware TC TCP : Ideal TCP Aggregates Target ates: Clent - 3: Mbps Clent - : X Mbps 6 Flows / Aggregate 0 Mbps Bottleneck Bandwdth (Mbps) 8.0 7.0 6.0 5.0.0 3.0.0.0 0.0 0.5.5.5 3.5.5 5.5 6.5 7.5 8.5 9.5 0.5.5 Target ate for TCP Aggregate between Clent and (X Mbps) TCP : Standard TC TCP : Target Aware TC TCP : Ideal TCP : Standard TC TCP : Target Aware TC TCP : Ideal TCP Aggregates Target ates: Clent - 3: Mbps Clent - : X Mbps 6 Flows / Aggregate 0 Mbps Bottleneck Target ate for TCP Aggregate between Clent and (X ms) Fgure 3: Acheved Bandwdth usng TATC-DP Fgure : Acheved Bandwdth usng TATC-3DP Fgure 3 shows the results of the experment wth standard TC and Target-Aware TC when the TATC-DP algorthm s used. The expected bandwdth s also plotted. It s observed that there s a gap n the expected and acheved bandwdth when standard TC s used. The excess bandwdth s not proportonally dstrbuted, as would be desred by a customer. Instead, we see an almost even dstrbuton of the excess bandwdth between two sets of competng flows. When the Tar- 0
get-aware TC s used, the acheved bandwdth s closer to the expected bandwdth for both the flow aggregates. Smlar results are shown n Fgure for the case when the TATC-3DP algorthm s used. In another experment, sx dfferent flow aggregates wth dfferent target rates are pushed through bottleneck lnks of 5 Mbps and Mbps. The total allocated target rate consttutes 0 % and 80% of the bottleneck lnk capacty respectvely. The experment s performed wth standard TC and TATC-3DP. Fgure 5 shows the acheved bandwdth for all aggregates n the case of the 5 Mbps bottleneck lnk. Fgure 6 shows the acheved bandwdth for all the aggregates n case of Mbps bottleneck lnk. It s seen that mprovement n bandwdth allocaton s sgnfcant for heavly over-provsoned network. The experment was repeated for the TATC-DP and the results closely resemble those n Fgure 5 and 6. Acheved Bandwth wth TATC-3DP - 0% Subscrpton Acheved Bandwth wth TATC-3DP - 80% Subscrpton 6.0 8.0.0 7.0.0 6.0 Bandwdth (Mbps) 0.0 8.0 6.0.0 Acheved BW: TC Acheved BW: TATC-3DP Expected BW 5 Mbps Bottleneck lnk 0 Mbps Host Edge Lnks Bandwdth (Mbps) 5.0.0 3.0.0.0 Acheved BW: TC Acheved BW: TATC-3DP Expected BW Mbps Bottleneck lnk 0 Mbps Host Edge Lnks.0 0.0 0.0 0.5.5.5 3.5.5 5.5 0.5.5.5 3.5.5 5.5 Target ates for Dfferent Aggregates (Mbps) Target ates for Dfferent Aggregates (Mbps) Fgure 5: TATC-DP 0% Capacty Allocaton Fgure 6: TATC-3DP 80% Capacty Allocaton Table reflects the extent to whch the dfferent Target-Aware TCs are able to acheve the expected bandwdth based on proportonal dstrbuton of the excess. The table shows the average devaton from expected value acheved by each traffc condtoner n each of the three experments performed n ths secton. For all three experments, t can be concluded that the standard TC has a hgher percentage devaton from expected results than ether of the TATC algorthms. The performance of TATC-DP and TATC-3DP are comparable. Table : Percentage Average Devaton from Expected esults. TC Fgures Fgures 5 & 6 3 & 80 % Provsoned 0 % Provsoned Standard 0.3 0.50 0.9 TATC-DP 0. 0.3 0.9 TATC-3DP 0.08 0.5 0. 7. Dscusson The prevous sectons have presented varous methods for ensurng a farer dstrbuton of bandwdth for flows n an AF-based Dffserv network. In ths secton, we evaluate the applcablty of the proposed solutons and dentfy the lmtatons. TT-Aware TC The TT-Aware TC has the followng requrements. Frstly, t s applcable for traffc streams where all flows n the aggregate have the same TT. Secondly, t requres the edge devces to determne the TT of aggregates passng through t. One possble way to do ths s to consder a
sngle flow as representatve of the aggregate. The edge devce can perform TT measurement of the aggregate traffc at the edge of the network. Ths wll requre per-flow state montorng of data packets and observng the return of correspondng ACKs n the reverse drecton. Such a scheme assumes that the delay from the edge to the host s mnmal. The thrd requrement s to determne the mnmum TT for aggregates n the network. Two approaches are possble. If queueng delay at core devces s mnmal then for pre-confgured pontto-pont connectons, the TT can be estmated based on the transmsson delay of ntermedate lnks. If, however, queueng delay s a major component n the TT, then the TT needs to be dynamcally measured. Thus, to determne the mntt, the edge nodes need to exchange TT nformaton co-operatvely. The analyss of the prevous secton showed that wth a large number of actve flows, the engneerng of ED parameters s also mportant to bandwdth assurance for Assured Forwardng based servces. However, a settng of large maxth s mpractcal snce t wll cause a large and varable end-to-end packet delay. Alternatvely, the number of actve flows at the core of the network can be lmted. Ths wll requre admsson control at the edge of the network. TCP/ Interacton The TCP/ studes (Fgure 0) show that usng drop precedence mappng, a certan level of farness can be acheved. Mappng TCP and n-profle traffc to DP0 helps to acheve the target bandwdth. However, mappng of TCP and out-of-profle traffc to dfferent drop precedence s necessary to handle the bandwdth dstrbuton at over-provsoned and underprovsoned states. Scenaro 6 satsfes the requred mappng and t s reflected n low percentage devaton n farness ndex (Fgure 0). As shown n Fgure 0, use of two queues to solate TCP and traffc provdes the optmum soluton. However, the approach has a possble drawback due to the necessty of knowng the fracton of TCP and target rates at the core of the network. Ths requrement can be handled by the use of bandwdth broker - to communcate target rates - or by pre-allocatng weghts for each queue based on an estmate of and TCP traffc. Target-Aware TC It s debatable whether the excess bandwdth n an over-provsoned network should be dvded among aggregates n proporton to the subscrbed target rates or should be dvded equally. Ths s a busness decson that shouldn t be nfluenced by techncal lmtatons. Should provders wsh to offer a proportonal dstrbuton of the excess, they should have the buldng blocks to do so at ther dsposal. The TATC-DP and TATC-3DP are two examples of such buldng blocks. Although the performance results of the TATC-DP and the TATC-3DP are comparable, there are practcal ssues to consder when evaluatng a Target-Aware TC. The TATC-DP wll ncrease the amount of n-profle traffc n the network. Ths makes traffc engneerng more dffcult because the total n-profle traffc cannot be estmated from the subscrbed total target-rates. On the contrary, the TATC-3DP has n-profle traffc that s consstent wth the target rates snce excess traffc s parttoned between DP and DP. Both the TATC schemes requre knowledge of the mnmum Target ate n the network. Ths s not as dffcult to obtan as the mnmum TT n the network. Target ates are typcally statc and don t change as often as TT. Thus, the mnmum Target ate can be perodcally determned va the exstng polcy management framework and communcated to the edge devces usng a COPS-lke protocol. Other Issues For over-provsoned networks, the target rates of aggregated flows are mostly achevable. As we have seen, the degree to whch the excess bandwdth s farly dstrbuted depends on varous factors. Dffserv SLAs wll lkely contan some form of quanttatve guarantee on performance
parameters such as bandwdth. It has been shown that varous factors play an mportant role n determnng the bandwdth obtaned by TCP and flows. As such, t s mportant to develop scalable approaches to ensure far dstrbuton of bandwdth that can be guaranteed n accordance wth SLA performance parameters. Though these parameters cannot be exact our work has shown that techncal solutons are feasble to provde some guarantee wth certan constrants. It s mportant to understand the scope (.e., topologcal extent) of servces for whch some form of quanttatve assurance can be gven. Varous traffc condtonng schemes may be feasble for all traffc between an ngress pont and an egress pont or a set of egress ponts. Further study s needed to address the scalablty ssues as the number of egress ponts ncrease. Developng traffc condtoners for a one-to-anywhere topology requres further work. 8. Conclusons The contrbutons of ths paper are the followng: (a) An ntellgent traffc condtoner to mtgate the mpact of TT on the acheved bandwdth for traffc aggregates wth equal target rates; (b) Possble approaches to address the farness ssues between TCP and traffc aggregates; (c) Two ntellgent traffc condtoners to dstrbute the excess bandwdth n over-provsoned networks n proporton to the target rates. The lmtaton of the above approaches are: (a) All the solutons assume one-to-one and one-tofew network topology (not one-to-any); (b) TT-Aware and Target-Aware ntellgent TCs are ted to TSW taggng algorthm. However, ths can be extended to other taggng approaches as well; (c) Edge nodes have to communcate among themselves to obtan certan state nformaton. Appendx A C * MSS The throughput of TCP flows can be represented by: A = () TT * p where C s a constant, A s the measured throughput, MSS s the packet sze, TT s the roundtrp tme and p s the packet drop probablty for that flow. The smplest objectve of the TT- Aware Traffc Condtoner s to ensure that two or more flows wth the same target rate and packet sze wll obtan an equal share of the excess bandwdth regardless of ther round-trp tmes. Consder two flows wth acheved rates A and A the objectve s to obtan: A = A () If the packet szes for the two flows are the same then the equaton reduces to: TT p = TT p (3) If the two flows have dfferent round trp tmes, then: p TT = () p TT Therefore, f we desre the same acheved rate for the two flows, the rato of ther packet drop probabltes should have an nverse squared relatonshp to the round-trp tmes. The TSW marker [3] operates as follows: f the measured rate of a flow s beyond ts target rate, t marks packets out wth the probablty p out : A T pout = q = (5) A We make the followng assumpton: 3
pout p (6) pout p That s we assume that the rato of out-of-profle packet markng s drectly proportonal to the rato of packet drop probabltes at the core. Ths s true because packets from both flows end up beng counted towards the same average queue calculatons n the core Therefore, for two flows at the edge,we modfy the out-of-profle markng scheme as follows: p out = q and p out TT = q TT (7) 3 TT If there was a thrd flow, t would be marked out-of-profle wth probablty: p out = q TT 3 (8) Based on the above, the markng scheme can be extended and generalzed to be applcable to n numbers of flows passng through the edge. TTmn The generalzed markng scheme for out-of-profle packets would be: pout = q TT (9) where TT s the round-trp tme for that partcular flow and TT mn s the mnmum round-trp tme of all the flows n the network. Ths dervaton has been developed for the case where each TCP flow has ts own target rate. However the dervaton can be extended to be applcable for TCP aggregates as long as all the flows n the aggregate have the same TT. 9. eferences [] Floyd, S., and Jacobson, V., andom Early Detecton gateways for Congeston Avodance, IEEE/ACM Transactons on Networkng, V. N., August 993, p. 397-3. [] Blake, S. Et al, "An Archtecture for Dfferentated Servces", FC 75, December 998 [3] Clark D. and Fang W., Explct Allocaton of Best Effort Packet Delvery Servce, IEEE/ACM Transactons on Networkng, V.6 N., August, 998 [] Ibanez J, Nchols K., Prelmnary Smulaton Evaluaton of an Assured Servce, Internet Draft, draft-banez-dffserv-assured-eval-00.txt>, August 998 [5] Henanen J., Baker F., Wess W., and Wroclawsk J., Assured Forwardng PHB Group, FC 597, June 999. [6] Jacobson V, Nchols K, Podur K, An Expedted Forwardng PHB, FC 598, June 999. [7] Ln W, Zheng and Hou J, How to Make Assured Servces More Assured, In Proceedngs of ICNP, Toronto, Canada, October 999. [8] Yeom, I and eddy N, ealzng throughput guarantees n a dfferentated servces network, In Proceedngs of ICMCS, Florence, Italy, June 999. [9] Maths M, Semske J, Mahdav J, Ott J, The macroscopc behavour of the TCP congeston avodance algorthm., Computer Communcaton evew, 7(3), July 997 [0] Morrs,., TCP Behavor wth Many Flows, In Proceedngs of IEEE Internatonal Conference on Network Protocols, October 997, Atlanta, Georga
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